All of Paul McCartney's compositions (written or co-written) that were released between 1969 and 2013 (also the Fireman ones) are in the book: a total of 383 sheets. All the other songs - including covers, many unreleased and live setlist - also find their place all over the volume, although without a specific sheet.

"Love is strange", for one, even if no info on Paul's recording of it, they could have listed the history of the song (and other covers), who wrote and first recorded it ect...

The main reason is well explained in the intro. Paul McCartney is a composer of popular music (and more recently, also classical) and thiI wanted to focus on his working method: from composing to recording to arranging his songs.
He will be remembered in the history of music for his compositions. Certainly through he years, he did cover a massive number of songs, as well, but I made a choice, that's all.
I made an effort also trying to give a glimpse through Paul's classical works, with two of the most prestigious interviews I've done (Carl Davis and Carlos Bonell).
There's ton of info and anecdotes in the book to be enjoyed. :)

... then why not also include the songs he's writen for other people ?

Not all of them are exactly masterpieces (then again, not all of his own songs are), but some, like ''Six O'clock'', are among his better compositions.

For the only reason that they were not part of his discography. I would have liked to throw in everything, believe me, but to analyze and write about "only" Paul's songs is quite a hard task. The scope of the book is to show and reveal how he works for his records, for his output. As an author, you have to set a limit to your work. But, being Paul McCartney's compositions, what a limit!!

Please note two things to complement Nick's comprehensive review, who received the book back in July:
- Recent reprint of the book (end September) is heavily improved, with corrections (suggestions courtesy of English writer Steve Lambley) and completely new layout.
- Volume II (containing covers, live material, unreleased songs, classical stuff) is in the works and planned for 2015.

Finally, there's a hardbound edition of the book available.
It's a strictly limited edition (200 copies), with flaps and cream-color paper. It was premiered at the Beatles Day in Mons, Belgium, last October, where I met with extreme pleasure a lot of friends and Mark Lewisohn.
The contents and the new internal layout mirror the last improved reprint released on October 1st, 2014 (the paperback one, which is currently available on all Amazon sites, including US).